Question

Do you have to use molar mass numbers if you are converting?

Answer 1

it depends on what you are converting. If you are converting between mass and moles, then yes. If you are converting between moles and ## of particles, then no.

Answer 2

and when you convert do you have to use the moles ratio first>

Answer 3

You can only use the mole ratio if you have moles already. For example:\[2Mg(s) + O{_2}(g)\rightarrow 2MgO(s)\] 2 moles of Mg will use 1mol of O2, but if you start with 12g of Mg, you need to convert to moles first, before finding how many moles of O2 will be used.\[12g Mg * (\frac{1mol Mg}{24.3g Mg}) = 0.5mol Mg\] NOW you can use the mole ratio of:\[0.5mol Mg * (\frac{1mol O{_2}}{2mol Mg}) = 0.25mol O{_2}\]